Although the need for flame-resistant fibers has been felt for many years, the available flame-resistant fibers have generally been less than satisfactory. Early examples of flame-resistant fibers were asbestos and fiber glass, followed by fibers of polyvinylchloride, the copolymer of vinyl chloride (60%) and acrylonitrile (40%), and polytetrafluoroethylene. These fibers are generally considered to be deficient in aesthetic properties when used in garments. More recently, fibers of poly(metaphenylene isophthalamide), MPD-I, have become available. Although the MPD-I fibers are quite satisfactory for many textile uses and exhibit good flame-resistant properties, they are relatively costly. Moreover, a need has been felt to provide flame-resistant textile products having a variety of other properties and aesthetic characteristics.
In order to make use of the various aesthetic properties of conventional, flammable fibers which have been on the market for many years, it has been proposed to blend them with flame-retardant fibers in proportions sufficient to make the blend nonflammable. It might be assumed that the burning propensity of a fiber blend of flammable and nonflammable fibers would decrease roughly in proportion to the amount of nonflammable fiber present, by dilution of the flammable material. The disclosure of U.S. Pat. No. 3,480,582 suggests that such a dilution principle may apply in the case of fiber blends used for carpets, which are heavy fabrics tested for flammability in horizontal position. This flammability test is a relatively mild one, since the burning gases rise from the fabric under test. Fabrics intended for apparel use are tested by a more stringent test in which the fabrics are mounted vertically and ignited at the bottom, so that the burning gases rise around and through areas of fabric not yet consumed by the flames. In this test, it is usually found that fabrics made of blends of flame-retardant fibers with conventional, flammable fibers actually burn more readily than fabrics made solely of the flammable fibers. Although there are various explanations for this phenomenon, one reason appears to be that the nonflammable fibers hold the flammable fibers in place, so that they cannot shrink away from the flame. Only when the nonflammable fibers constitute a very high proportion (i.e., about 85% or higher) of the total material present is the blend found to be nonflammable. From the aesthetic viewpoint, fabrics containing such a high proportion of the nonflammable fiber are usually essentially equivalent to fabrics made solely of the nonflammable fiber. Accordingly, it has been desired to make fabrics containing higher proportions of the conventional, flammable fibers.